In a recording system, the signal read from a disk is first amplified by a pre-amplifier. The signal then passes through an analog front end (AFE) before it is sampled by an analog-to-digital converter (ADC). The AFE includes a variable-gain amplifier (VGA) to scale the signal range at the input of the ADC to effectively utilize the full range of the ADC; otherwise, quantization noise and saturation of the output of the ADC can severely degrade the performance of the system.
In some magnetic recording systems, the information stored in a sector of a disk includes an acquisition portion and a user data portion. In some embodiments, the acquisition portion is a preamble. The preamble precedes the data portion, and the preamble's data pattern is known. For example, the preamble may comprise repetitions of the bit pattern [+1 +1 −1 −1]. Some systems may have a preamble gain-loop to adjust the VGA gain during the preamble and a tracking gain-loop to adjust the VGA gain during the user data portion of the sector. The preamble gain-loop adjusts the VGA gain such that the preamble amplitude at the input of the ADC converges to a targeted value, i.e., the preamble target amplitude At. Existing techniques for determining the preamble target amplitude At have a number of drawbacks. Thus, improved techniques for determining the preamble target amplitude At would be desirable.